This invention relates to turnaround control for mechanically scanned radar antennas.
"A tracking radar must first find and acquire its target before it can operate as a tracker. Therefore it is usually necessary for the radar to scan an angular section in which the presence of the target is suspected. Most tracking radars employ a narrow pencil-beam antenna. Searching a volume in space for an aircraft target with a narrow pencil beam would be somewhat analogous to searching for a fly in a darkened auditorium with a flashlight. It must be done with some care if the entire volume is to be covered uniformly and efficiently." (M. I. Skolnik, "Introduction to Radar Systems" Second Edition, 1980. McGraw Hill Book Co., page 177). Examples of scanning patterns are shown by Skolnik on page 178. The raster scan is a simple and convenient means for searching a limited sector, rectangular in shape, in a uniform manner. It is also called an n-bar scan, where n is the number of horizontal rows. "Onbar" is a period during which the elevation remains fixed while the azimuth varies from one end of a bar to the other end. During an offbar or turnaround period the elevation changes from one bar to another, preferably in a smooth curve in which the azimuth continues to change out to a limit and then returns to the beginning of the next bar.
Antenna pickoffs are used in many systems to provide an accurate determination of antenna position. However they are expensive and are therefore omitted in some cost-sensitive systems. In the latter systems, the antenna turnaround mechanization may be a source of degraded system performance and reduced system flexibility. A turnaround must be at the end of each bar in a scan pattern, which introduces error transients into the true antenna pointing vector, and they extend into the "onbar" period. The two factors which have most impeded a solution to this problem are (1) requirements for frequent, variable length, system calibrations, and (2) the lack of antenna pickoffs for accurate determination of antenna position.
One previous system filters a scan rate reversal and them integrates to generate a smooth turn. To minimize transients, this filter runs during "onbar" also, producting a slowed scan rate and increased frame time. The scan generated vector is used as the true vector, which results in errors as large as 0.5-0.7 degree, during onbar. Much larger errors exist during the turn, thus, this time is used for calibrations ands tests instead of radar data collection. This "blind time" is reduced by setting the scan filter coefficients to generate a constant 200-millisecond turn. In those cases in which the calibration does not finish, the control software attempts to stop the antenna, introducing worse transients in the onbar period.
Other previous systems did not have requirements for frequent calibrations and did have antenna pickoffs. Two of these systems did generate the turnaround by an integrated, filtered, scan rate reversal and did not perform standard data collection during the turn period. In another of these systems, the turn was generated in an open loop manner using an AZ-EL independent rate/acceleration limiter to smooth an instaneous jump to the start of the next bar. This technique generates a constant turn period, with no control of excursion, but the correct onbar scan rate was achieved.
In previous systems, the antenna was controlled by rate only, and not by position. The control was by error signals. There was no actual control of the path. The error signals gave no useful indication of the antenna position. There was a serious problem with gimbal limits.